I'll second the Anti-vir. been using it for years, and never had any issues.
It is not the prettiest thing, but work it does. If you get a busy month, you could see 2 or 4 complete program updates in a couple of weeks. biggest downside is that it does require user intervention to install the updates, this is where avg has an edge. It is one of the top rated anti viruses, ahead of norton ,macafee and a few other pay programs.
For your slow system, after you do all the scans with adaware, spybot, and such, I would also suggest you open spybot in advance mode, go to tools, and look at the startup list. if you didi the full install, it should give you description of most items in the startup list, and it includes the registry entries too. What happens is everything lately wants to start some systray applet on install, and it all impacts startup. You don;t need
quicktime, realplayer, aim, office quickstart and many other applications
that get shoved in there, and they all slow down your startup. If unsure tho, leave it and google it up, there are a couple of sites that have some pretty comprehensive lists of what is what in the startup list.
IMHO, tho, if you can spare 10 gig, make the partition, and isntall xp on it. If you formatted it with the parittioning software, xp will ask if you want to format, if not it will do a chkdsk on reinstall.
This second partition could be very handy if you have any problems on your main xp setup and need to fix something or recover data.
One other thing, you can reinstall xp over itself. you will not use yoru programs, or shortcuts, it will just revert xp to stock. if this is a pre sp1 or sp2 cd, you'll be doin ALOT of updating to get back to where you were. I;ve done this a few times to recover people's pc's, and while not the best route, if they have tons of stuff installed, it is often the path of least resistance. I would only recommend going that route tho if the os itself is damaged and won;t run. In your case, some patient cleaning should bring the system back to life.